摘要：用戶在調用被動態代理過的對象的方法時，調用的請求會被自動發給代理對象實現了接口的方法，由方法來實現對請求的統一處理。動態代理實現切面代替該語句塊一定會在前面執行。
用戶在調用被動態代理過的對象的方法時，調用的請求會被自動發給代理對象（實現了InvocationHandler接口）的invoke()方法，由invoke()方法來實現對請求的統一處理。
源碼分析
Java.lang.reflect.Proxy
public class Proxy implements java.io.Serializable {
private static final Class[] constructorParams = { InvocationHandler.class };
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
......
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class[] intfs = interfaces.clone(); //淺克隆
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
//生成代理類的class
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//getConstructor(InvocationHandler.class) 獲取代理類的構造器
final Constructor cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//代理類實例化返回
return cons.newInstance(new Object[]{h});
}
......
}
private static Class getProxyClass0(ClassLoader loader, Class... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// key為WeakReference，若緩存中存在相同key，則返回ProxyClass，若不存在，則用ProxyClassFactory創建ProxyClass。
return proxyClassCache.get(loader, interfaces); //java.lang.reflect.WeakCache 這個內核分析？
}
......
}
private static final class ProxyClassFactory implements BiFunction[], Class> {
//?所有代理類ProxyClass名字的前綴?
private static final String proxyClassNamePrefix = "$Proxy";
// 用計數器生成代理類ProxyClass的名字
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class apply(ClassLoader loader, Class[] interfaces) {
Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//對每一個interface Class進行驗證
for (Class intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
* 用IdentityHashMap來驗證interface Class的唯一性，因為IdentityHashMap的key比較是==，即比較內存地址。
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null;
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
// 對于非公共接口，ProxyClass的包名設置為接口的包名??
for (Class intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf(".");
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
//?對于公共接口，ProxyClass的包名為com.sun.proxy??
if (proxyPkg == null) {
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
long num = nextUniqueNumber.getAndIncrement();
//ProxyClass的完全限定名
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//由c++寫的ProxyGenerator生成代理類的二進制字節碼
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//?根據二進制字節碼返回相應的ProxyClass??
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
}
$Proxy.class
以下是反編譯$Proxy0.class：
public?final?class?$Proxy0?extends?Proxy?implements?MyInterface1{??
??private?static?Method?m1;??
??private?static?Method?m3;??
??private?static?Method?m0;??
??private?static?Method?m2;??
?
??public?$Proxy0(InvocationHandler?paramInvocationHandler)?{??
????super(paramInvocationHandler);??
??}??
??
??public?final?boolean?equals(Object?paramObject)?{??
????try?{??
??????return?((Boolean)this.h.invoke(this,?m1,?new?Object[]?{?paramObject?})).booleanValue();??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??public?final?void?sayHello(String?paramString)?{??
????try?{??
??????this.h.invoke(this,?m3,?new?Object[]?{?paramString?});??
??????return;??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
???catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??public?final?int?hashCode()?{??
????try?{??
??????return?((Integer)this.h.invoke(this,?m0,?null)).intValue();??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??public?final?String?toString()?{??
????try?{??
???? return?(String)this.h.invoke(this,?m2,?null);??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??static?{??
????try?{??
??????m1?=?Class.forName("java.lang.Object").getMethod("equals",?new?Class[]?{?Class.forName("java.lang.Object")?});??
??????m3?=?Class.forName("com.mikan.proxy.MyInterface1").getMethod("sayHello",?new?Class[]?{?Class.forName("java.lang.String")?});??
??????m0?=?Class.forName("java.lang.Object").getMethod("hashCode",?new?Class[0]);??
????m2?=?Class.forName("java.lang.Object").getMethod("toString",?new?Class[0]);??
??????return;??
????}??
????catch?(NoSuchMethodException?localNoSuchMethodException)?{??
??????throw?new?NoSuchMethodError(localNoSuchMethodException.getMessage());????}??
????catch?(ClassNotFoundException?localClassNotFoundException)?{??
??????throw?new?NoClassDefFoundError(localClassNotFoundException.getMessage());??
????}??
??}??
}?
?
$Proxy0.class的總結：
對于要代理沒有接口的類，則必須繼承該沒有接口的類，從而覆蓋其方法，但由于java不能多繼承，這里代理類已經繼承了Proxy類了，不能再繼承其他的類，所以JDK的動態代理不支持對實現類的代理，只支持接口的代理。
構造方法的入參為InvocationHandler，這說明該代理類是實現了InvocationHandler接口的類的代理類，而InvocationHandler實現類再代理真正的實體類。
靜態代碼塊會初始化真正實體類的Method對象，以及Object類的equals、hashCode、toString的Method對象。
總結：
Proxy 會從內部類WeakCache中根據ClassLoader 和interfaces獲取ProxyClass（若緩存中存在相同key，則返回ProxyClass，若不存在，則用ProxyClassFactory創建ProxyClass（由ProxyGenerator生成二進制碼，再將寫成*.class文件））。
AOP JDK動態代理實現
切面InvocationHandler：
public?Object?invoke(Object?proxy,?Method?method,?Object[]?args)?throws?Throwable?{??
????//?BeforeAdvice??
????Object?retVal?=?null;??
????try?{??
????????retVal?=?method.invoke(target,?args);??//?AroundAdvice?代替該語句?????
????????//?AfterReturningAdvice??
????}??
????catch?(Throwable?e)?{??
????????//?AfterThrowingAdvice??
????}??
????finally?{??
???? //finally塊一定會在return前面執行。
????????//?AfterAdvice??
????}??
????return?retVal;??
}??
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